The measurement and monitoring of low volume fluid flows has various applications including applications in industrial and residential settings. For example, in the chemical industry the accurate and precise knowledge of inlet and outlet flows for a myriad of processes (e.g. chemical reactions) can be critical to the optimal production and processing of chemicals, pharmaceuticals and the like. Precise monitoring of flows can also be used to discover and prevent leaks which can be costly and be a safety issue.
Additionally, the lack of low-flow monitoring can result in losses to the suppliers of such flow. For example, water companies are compensated for water usage as measured by their flow monitors (water meters). If their flow monitors do not measure trickle or drip flow, they are not reimbursed for such usage. The loss of revenue can be considerable. Additionally, the location of the loss is not detected thereby allowing a large amount of water to be wasted. This is particularly an issue in the many countries with limited water supplies. Furthermore the knowledge of this monitoring limitation can be used to steal water, for example by slowly dripping water into a holding tank, at a rate not measurable by the associated flow meter, and consuming the water directly from the tank.
Turbine flow meters, which are the conventional magnetic flow meters in general use today have long been used to measure fluid flow by means of a turbine immersed in the fluid. A magnet connected to the turbine turns a second magnet, which is placed in a dry area. The second magnet drives a cog system that turns a mechanical counter. These flow meters are unable to detect low flows e.g. below about 10 l/h when considering a typical water meter of the type installed by water supply companies and municipalities world wide. Positive displacement metering devices are also commonly used to measure flow rate and they have deficiencies in particular where water is of poor quality i.e. has a high calcium content or contains dirt such as sand.
Other types of flow meters are also known, some of which are devices for measuring low volumetric fluid flow. However such meters are typically costly, require servicing and are difficult to retrofit, thus are usually not used for domestic water metering.
Droplet counter devices are also known, wherein a sensor is provided for droplet count. However, such devices usually service for laboratories and are not cost-effective in massive installation, e.g. for use by a water supply company, certainly not for urban use. Even more so, such systems are not easily retrofitted and they require some considerable space.
For example, disclosed in U.S. Pat. No. 5,218,346 to Meixler is a low volume flow meter for determining if a fluid flow meets a minimum threshold level of flow. The monitor includes an externally located electrical portion, which operates with a minimum of intrusion to the flow and allows for repairs. The electronics provide for the adjustment of the threshold level and can be modified to provide for a parallel electronic circuit for a bracketing of the desired flow rate. However, the system is not simple or inexpensive.
Another type of flow rate device that has the capacity to measure or monitor a low flow rate is a compound meter. In this case, the device comprises a high flow metering device together with a secondary flow meter that is typically located in a by-pass conduit. There is typically some means for diverting flow (e.g. by using a “change-over” valve set to activate at a pre-determined pressure) based on a pre-determined flow rate or pressure in order to direct the flow to the appropriate meter. These meters typically suffer from at least some of the above-mentioned drawbacks and in particular are expensive.
A problem which may occur with flow metering devices is so-called ‘over-efficient’, where the flow meter may read excessive amounts of fluid, which in fact have not flown through the system. This may result for example, owing to inertial revolutions of the measuring impeller of the metering device.